Alex Millhouse was chatting on a pay phone at the Mobil gas station in Aspen Hill when he thought he heard a car backfire. He glanced up and saw a cabdriver standing next to a gas pump a few feet away, clutching his side as blood streamed down his shirt and pants.

"I gotta go," Millhouse told his friend on the phone. "Someone has been hurt."

Millhouse, a mechanic at the gas station, scrambled over to the pumps, where the dying cabby stumbled backward a few steps before collapsing onto the pavement.

The mechanic yelled for someone to call 911. A female customer began performing CPR. Blood quickly pooled around the middle row of gas pumps as the 54-year-old cabby, identified as Premkumar A. Walekar of Olney, stopped breathing.

The tail end of rush-hour traffic zipped through the congested intersection of Connecticut Avenue and Aspen Hill Road before the police cruisers and ambulance arrived, sirens blaring. It was about 8:15 a.m.

The cabdriver was not the first person gunned down that morning along the busy commercial corridors of southern Montgomery County, nor would he be the last.

Like the scene of the four other fatal shootings in a 17-hour rampage, the Aspen Hill Mobil mart was bustling with bystanders and witnesses when the shooter pulled the trigger.

Despite the crowds, the culprit apparently managed to avoid standing out, slipping away each time without attracting notice. There were no visible confrontations, no words spoken.

Millhouse said he couldn't tell where the bullet came from. Nor did he see anyone running or driving away. "I saw nobody. No cars speeding, nobody running, nothing. All there was was a loud boom."

Police said the first shot may have been fired at 5:20 Wednesday evening, when a bullet pierced the window of a Georgia Avenue store. But the killer struck his first target about 6 p.m. Wednesday, outside the Shoppers Food Warehouse in Glenmont.

The grocery store's parking lot was full, with cars lined up on an access road waiting for a free space. James D. Martin, a 55-year-old federal employee from the Colesville area, was walking in the lot close to the exit to Randolph Road when he was gunned down.

Inside a brick district police station across the street, officers jumped up when they heard the shot. "It was loud enough that everyone went running out to see what it was," Lt. Demitri Kornegay said.

Many shoppers didn't realize what had happened. One customer told Belinda Miller, a cashier, that she had walked past Martin's body on the pavement. "She thought he was under his car working on it," Miller said.

Police secured the scene and interviewed passersby, but they had no way of knowing that they would have to repeat their actions several more times over the next 16 hours.

The next slaying would not come until the following morning. About 7:40 a.m., the shooter was watching James L. "Sonny" Buchanan Jr. mow the grass at a car dealership near Rockville Pike, perhaps the county's busiest commercial corridor.

Buchanan pushed his lawn mower over a strip of turf on Huff Court, a short street bordering a parking lot at the back of the dealership. A single shot was fired.

"It was real loud," said Gary Huss, body shop manager at the dealership, the Fitzgerald Auto Mall. "I thought maybe a very large solid object had fallen on the ground."

Moments later an employee who was in the back parking lot saw Buchanan stagger through a gate in the fence, clutching his chest and gasping for air, Huss said. Buchanan made it about 200 feet into the lot before collapsing face forward onto the ground. The employee began screaming for someone to call 911.

Within seconds, about 100 workers from the dealership had gathered around Buchanan, who was unconscious and bleeding profusely.

It did not immediately dawn on anyone that they had stumbled onto a crime scene. Huss said he figured Buchanan had been hit by something his lawn mower sucked in.

He ran out to inspect the machine and found nothing amiss. "That's when my suspicions went up," he said.

Emergency personnel arrived about 10 minutes later. They turned Buchanan over to reveal a gaping wound in his chest, just below his heart.

Dottie Fitzgerald, president of Fitzgerald Auto Malls, gave police videotapes recorded by surveillance cameras mounted on the dealership's building. But employees said the cameras' range did not extend to the spot where Buchanan was standing and a cursory review of the tapes had not shown anything out of the ordinary.

The next victim was the cabdriver, in Aspen Hill. The shooter then turned due north, police said, to Leisure World, a giant retirement community that is home to some 7,000 senior citizens.

Sarah Ramos, 34, was sitting on a metal bench outside the post office there when the killer took aim and fired.

Ramos was sprawled on the bench, bleeding from the head, when Dolores Wallgren walked by on her way to an appointment at a beauty shop. "She was sitting on the bench, just sitting there," Wallgren said.

Police quickly roped off five businesses: a diner, a carpet store, a health food store, a chicken take-out restaurant and the post office. Many elderly bystanders -- some pushing walkers or holding canes -- were so unfamiliar with crime scenes in their neighborhood that they tried to walk under the yellow police tape before they were shooed away by officers.

Carmen Hunt, an employee of a HoneyBaked Ham store nearby, broke down and cried as she stared at the bench where Ramos was slain. "Working in this area you would never expect something like this to happen here," she said.

The scene repeated itself about 75 minutes later at a Shell station in Kensington, about a mile south on Connecticut Avenue.

Jimmy Akca, a mechanic on duty, was working under the hood of a green BMW in one of the service bays when he heard a loud bang. He saw a woman pinned under the door of a burgundy minivan a few yards away.

"People were yelling, 'Call the police, call the police,' " Akca recalled. "I saw the woman lying down. The door to the van was open. I thought she had a heart attack. But then I saw blood in her mouth and nose. Her eyes were rolling back."

The woman, Lori Lewis Rivera, 25, had been vacuuming her vehicle next to the service station, visible from all directions at the intersection of Knowles Road and Connecticut Avenue, in the heart of Kensington.

Again, however, there was no sign of the shooter. Within a half-hour, dozens of Montgomery police officers had formed a large perimeter around the Shell station. They fanned out across the parking lot of a nearby Safeway store and searched a large grassy area but did not report finding any evidence.

Lewis Rivera was one of many customers who had stopped at the Shell station, but the shooter made her the target.

"She just pulled in to vacuum her car," Akca said. "It's scary. Very scary."